This application is based on and claims priority from European Patent Application 01309439.6 filed on Nov. 7, 2001, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a lithographic projection apparatus.
2. Description of Related Art
The term xe2x80x9cpatterning structurexe2x80x9d as here employed should be broadly interpreted as referring to a structure that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning structure include:
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. The required matrix addressing can be performed using a suitable electronic structure. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning structure as hereabove set forth.
A lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
To reduce the size of features that can be imaged, it is desirable to reduce the wavelength of the projection beam of radiation. It has been proposed to use wavelengths of less than about 200 nm, for example 193 nm, 157 nm or 126 nm. Further reductions in the wavelength to the range of EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range of 5-20 nm) are envisaged. Such wavelengths in particular are more conveniently focused and controlled by reflective optics, such as mirrors. However, mirrors in lithography apparatus must be positioned to especially high accuracy, as compared to refractive elements, because any rotational orientation errors are magnified by the total downstream optical path length. In any apparatus using very short wavelength radiation, the optical path length may be of the order of 2 m or more.
For example, to have a good overlay performance, it can be necessary to keep the position of an image of an irradiated portion of the mask stable at a given position at substrate level with an error (e) of less than about 1 nm in particular when EUV is used. If the distance between the mirror and the substrate is 2 m the maximum permissible rotational error of the reflected beam, to keep the system within specification, is 28xc3x9710xe2x88x929 degrees (1xc3x9710xe2x88x929 m/2 m=tan (28xc3x9710xe2x88x929 degrees)), if e=1 nm. Since, for a mirror, the angle of reflection equals the angle of incidence, a rotational error in the position of the mirror will give rise to twice as large an error in the direction of the reflected beam. Thus, the mirror must be positioned with an accuracy of 14xc3x9710xe2x88x929 degrees or better. If the mirror has a width in the order of 0.1 m and a rotating point at one side, that rotating point must be positioned to within 0.024 nm (tan 14xc3x9710xe2x88x929xc3x970.1=2.4xc3x9710xe2x88x9211). Clearly, the accuracy with which such a mirror must be orientated is extremely high and will only increase as the specification for image accuracy increases. The accuracy requirements for position in X, Y and Z are less demanding, as such errors are magnified less at substrate level, but still remain high.
The projection system of a lithographic projection apparatus that is used in xe2x80x9cscan modexe2x80x9d (the meaning of xe2x80x9cscan modexe2x80x9d has been explained before and will be explained hereafter with reference to the accompanying drawings) and that uses EUV may include six mirrors, for example, for reflecting and thereby projecting the patterned beam onto a target portion of the substrate. In this case, the mirrors are to be positioned relative to each other with an accuracy of about 0.1 nm. It has been proposed before to use a plurality of actuators for adjusting the position and/or orientation of a reflective optical element. For example, a corresponding arrangement is described in EP 1107068 A2. This document describes the use of position sensors to maintain the reflective element stationary in spite of vibrations that might occur.
An optical member generally has six independent degrees of freedom (DOF), three transational and three rotational DOF. One possibility to adjust the optical member with respect to more than one DOF is to use a plurality of actuators. The actuators may be piezoelectric, electroresistive or magnetoresistive and act, for example, perpendicularly to a surface of the optical member which extends transversely to the beam of radiation incident at the optical member. In particular, when the desired maximum position error is in the region of 1 nm or smaller, the combination of the actuators or the combination of at least one actuator with other moveable devices, such as a gravity compensator, might deform the optical member, for example due to excessive and/or undesired differential forces.
It is an aspect of the present invention to provide a lithographic projection apparatus having at least one optical member wherein the apparatus comprises a structure for reducing or diminishing deformations of the optical member.
This and other aspects are achieved according to the invention in a lithographic apparatus including a radiation system that provides a projection beam of radiation; a support structure that supports a patterning structure, the patterning structure serves to pattern the projection beam according to a desired pattern; a substrate table that holds a substrate; and a projection system that projects the patterned beam onto a target portion of the substrate. At least one of the radiation system and the projection system comprises at least one optical member. The optical member is supported by and connected to a mounting frame which at least partially encloses the optical member and which is connected to at least one other component of at least one of the radiation system and the projection system.
The optical member is for example a mirror or a lens. In any case, the mounting frame shields the optical member against undesired influences which might be caused and/or transferred by the at least one other component of the lithographic projection apparatus to which the frame is connected. In particular, the frame can significantly reduce deformation of the optical member due to the frame""s stiffness.
According to a preferred embodiment, a connection between the frame and the optical member comprises a membrane-like portion located in a cut-out area of the frame and held by the frame at its outer circumference. The optical member is connected to a central area of the membrane-like portion. For example, the membrane-like portion extends within a plane including its outer circumference and the central area where it is indirectly or directly connected to the optical member. Preferably the outer circumference extends along a circular line. In particular, if the membrane-like portion is a thin sheet of material, the membrane-like portion may be circular. However, it is most preferred that the membrane-like portion comprises a plurality of spokes which extend from the central area to the outer circumference. The spokes may be separate parts of the membrane-like portion or may be connected by at least one further part of the membrane-like portion, for example by a ring-like part at the outer circumference and/or in the central area.
Generally, the membrane-like portion decouples the optical member from the frame with respect to deformations of the frame that might occur in spite of the frame""s stiffness. In particular, the membrane-like portion may deform in a first direction perpendicular to a second direction in which the membrane-like portion extends. Thereby the membrane-like portion can compensate a deformation of the frame in a direction opposite to the first direction. As a result the optical member stays in place, without deformation or displacement. The membrane-like portion is also able to decouple translation of the frame along a direction perpendicular to the plane of the membrane-like portion. In case the membrane-like portion is sheet-like, it can decouple the optical member from the frame regarding deformations of the frame in at least (but not only) two rotational degrees of freedom (DOFs) namely the DOFs with respect to two perpendicular rotational axis extending within the plane in which the sheet-like portion extends.
In case the membrane-like portion comprises the spokes, it can decouple the optical member even regarding the rotational DOF with respect to the rotational axis that extends through the center of the spokes perpendicularly to a plane in which and/or along which the spokes extend.
The decoupling regarding rotational DOF is particularly useful, if the membrane-like portion is used for connecting the optical member to the frame in order to provide enough stiffness in one or two linear DOFs in which the optical member is to be moved or displaced, but to avoid undesired displacement in the other DOFs. For example, an actuator or an arrangement of plural actuators which causes displacement in the desired DOF might also cause undesired displacement or deformation of the frame in the other DOFs
In a particular embodiment, the frame comprises a plurality of struts which may extend within a plane of the frame within which the optical member is enclosed by the frame. The struts are connected at corners of the frame so as to form a frame structure with at least three corners. Preferably, the frame structure has exactly three corners where the struts are connected. According to this embodiment, the frame is particularly stiff and light.
If the struts extend within a plane, the stiffness with respect to undesired forces acting from the outside of the frame in one of the directions of the plane is particularly large. On the other hand, the planar structure allows radiation to be incident at the optical member in a wide sector of directions transverse to the plane in which the struts extend.
The frame structure may comprise a corner element which connects two of the struts at one of the corners of the frame structure. In particular, the corner element may serve to establish a connection of the frame to the at least one other component of the apparatus. For example, the corner element may be an actuator or part of an actuator for adjusting and/or amending the position of the optical member.
The stiffness of the frame can be further improved by a stiffening member for stiffening the frame structure wherein the stiffening member extends between two of the struts which are connected at one of the corners so as to connect the two struts and so as to leave a hollow space in between the two struts, the stiffening member and the corner. Also, it is possible to provide pairs or groups of stiffening members located at the surface of the same hollow space. Further, the stiffness can be improved by providing a member which fills the angle between the two struts in the region of the corner where the two struts are connected.
The outer configuration of the struts may be plate-like which means that the outer dimensions are equal to the dimensions of a plate. The struts may be solid or hollow, in order to reduce the weight. The plate-like struts may each extend between two of the corners wherein the widths of the struts in a direction perpendicular to said plane are greater than a thickness of the struts measured in said plane in a direction perpendicular to a line which connects the respective two corners. This construction is light and stiff, not only with respect to direction in the plane of the frame structure, but also with respect to directions transverse to this plane.
Preferably, two struts which are connected to each other have equal lengths and are arranged symmetrically to each other. Most preferably, all side lengths of the frame structure between the corners are equal. A regular and symmetric structure comprises equal stiffnesses with respect to different directions and is particularly stiff.
In particular, if the frame structure has equal side lengths, the optical member is preferably connected to the frame structure at respective locations half way between each two of the corners. Especially for regularly and/or symmetrically shaped optical members, this embodiment enables to keep the lengths of the connections between the frame structure and the optical member short. A short connection is less likely to deform with large amplitudes. Further, the equally sided frame can effectively decouple the optical member from the at least one other component of the apparatus with respect to deformations, in particular when the other component is connected to the frame at one of the corners.
Preferably the frame is symmetric to the plane in which the frame extends. Furthermore, it is preferred that the optical member is arranged symmetrically to the plane. Generally, a symmetric arrangement is more resistant to undesired deformations, for example due to changes in temperature.
In one embodiment of the invention, at least one actuator for adjusting the position and/or the orientation of the frame and thereby of the optical member is connected to the frame. Preferably a direction or rotational axis of an actuation caused by the actuator is aligned with an axis of symmetry of the frame and/or the actuation direction or rotational axis is part of the plane in which the frame extends. In particular, the connection of the actuator to the frame is established at a location which includes a point or area within said plane.
It is a further aspect of the invention to provide a lithographic projection apparatus manufacturing method including providing a radiation system that provides a projection beam of radiation; providing a support structure adapted to support a patterning structure, the patterning structure serving to pattern the projection beam according to a desired pattern; providing a substrate table adapted to hold a substrate; providing a projection system that projects the patterned beam onto a target portion of the substrate; providing at least one optical member; and supporting the optical member by connecting the optical member to a mounting frame which at least partially encloses the optical member and which is connected to at least one other component of at least one of the radiation system and the projection system.
According to a further aspect of the invention there is provided a device manufacturing method including providing a substrate that is at least partially covered by a layer of radiation-sensitive material; providing a projection beam of radiation using a radiation system; endowing the projection beam with a pattern in its cross-section; and projecting the patterned beam of radiation onto a target portion of the layer of radiation-sensitive material using a projection system. At least one of the projection beam and the patterned beam passes through at least one optical member that is supported and is partially enclosed by a mounting frame which is connected to at least one other component of at least one of the radiation system and the projection system. The frame extends in a plane transverse to a path of radiation to or from the optical member and the frame thereby enclosing, within in said plane, the optical member, wherein the optical member is connected to the frame and the frame is connected to at least one of the other components.
In a preferred embodiment, the method includes adjusting the propagation direction of the projection beam of radiation or of the patterned beam by adjusting at least one of the position and the orientation of the mounting frame, thereby adjusting at least one of the position and the orientation of the optical member. Due to the stiffness and/or the decoupling effect of the mounting frame that are described above, the position and/or the orientation of the mounting frame can be kept very stable. For example, it is therefore possible to reduce the dimensions of structures which are generated by lithography on a substrate, in particular when short-wave radiation is used, e.g. EUV.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range 5-20 nm), as well as particle beams, such as ion beams or electron beams.